1. Field of the Invention
The present invention relates to an electronic part represented by a so-called multilayer wiring substrate produced by laminating metal foils on which resin is attached on both sides of a substrate and a method of manufacturing such an electronic part. More specifically, the present invention relates to a laminated substrate having excellent surface evenness that makes it possible to apply thereon post-processing such as polishing effectively and a method of manufacturing such a laminated substrate.
2. Related Background Art
An electronic part called a multilayer wiring substrate was developed in order to achieve high density in electronic apparatus. In the multilayer wiring substrate, various wiring which used to be provided on a printed circuit substrate is provided inside a laminated substrate to make it possible to achieve high density mounting. In a concrete manufacturing process, wiring and electrodes etc. are formed on the upper and lower surfaces of a substrate as a base member, a copper foil on which a resin layer as an insulating material is formed (that is, a resin coated copper, which will be referred to as RCC hereinafter) is laminated on the upper surface of the wiring and the electrodes etc. with the resin layer side facing that surface, the wiring and the electrodes etc. and the copper foil are brought into electrical contact and space formed between the substrate and the RCC is filled with the resin simultaneously, and thereafter a wiring pattern is formed on the copper foil. A multilayer wiring substrate inside which insulating layers having wiring etc. can be produced by performing the above described process repeatedly.
As disclosed for example in Japanese Patent Application Laid-Open No. 2003-124603, in the RCC lamination process, the RCC is not in direct contact with the substrate when it is pressed by pressurizing means for pressing. In the actual press process, an intermediate metal plate made of a stainless steel or the like and a buffer member in the form of a paper or silicon rubber sheet etc. is inserted between the RCC and the pressurizing means. Thanks to the presence of such members, more uniform pressure is effectively applied on the entire surface of the RCC. In connection with this, the buffer member is required to have cushioning properties, durability, releasability from a hot platen in some cases as mentioned in the above document and, in addition, sometimes required to have fluidity as disclosed in Japanese Patent Application Laid-Open No. 10-190225.
With downsizing and improvement of electronic parts in recent years, evenness of the surface of multilayer wiring substrates (or uniformity in the substrate thickness) has been demanded. Especially in the case where the multilayer wiring substrate is caused to function as a high precision electronic part by controlling the dielectric constant of the insulating layer or other factors, it is known that several micro meter variations of the insulating layer result in variations in properties of the electronic part. In view of this, methods of subjecting the substrate surface to processing such as polishing after completion of lamination to uniformize the substrate thickness mechanically have been adopted depending on the use.
For example, when a multilayer wiring substrate is produced by the method disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-124603, it has been found that a bulge of a resin layer or a region in which the thickness of the insulating layer increases unevenly is formed in the neighborhood of the outer circumference of the substrate. The presence of such a region causes a difficulty in using the multilayer wiring substrate as it is after pressing and, in addition, affects the polishing process for making it even. In other words, it may lead to a difficulty in determining the reference surface in polishing and in improving the precision of polishing above a certain level and a decrease in the processing efficiency due to an increase in the polishing amount. Furthermore, when the number of laminated layers is increased, there is a possibility that the risk of positional displacement upon lamination increases if the lamination is performed on such a substrate as it is, or the laminating pressure is not applied on the substrate uniformly.